Routing protocol based redundancy design for shared-access networks

ABSTRACT

A protection CMTS is available to immediately service a cable modem should that modem&#39;s service from a working CMTS fail for any reason. To speed the service transfer (cutover) from the working CMTS to the protection CMTS, the cable modem may preregister with the protection CMTS well before the cutover becomes necessary. The cable modem&#39;s registration with both the working CMTS and the protection CMTS preferably employs a single IP address, so that the cable modem need not obtain a new IP address during cutover. While the cable modem may register with both the working CMTS and the protection CMTS, the devices are designed or configured so that only the working CMTS injects a host route for the cable modem into the appropriate routing protocol. Only after cutover to the protection CMTS does the protection CMTS inject its host route.

CROSS-REFERENCE TO RELATED APPLICATION

This invention is related to U.S. patent application Ser. No.09/484,611, filed on the same day as this patent application, naming F.Daruwalla, J. Forster, G. Roeck, J. Chapman, J. Zang, and Y. Lu asinventors, and titled “METHOD FOR A CABLE MODEM TO RAPIDLY SWITCH TO ABACKUP CMTS.” This application is incorporated herein by reference inits entirety and for all purposes.

BACKGROUND OF THE INVENTION

This invention relates to digital cable network technology. Morespecifically, it relates to methods and apparatus that provideredundancy for critical headend components of digital cable networks.

Broadband access technologies such as cable, fiber optic, and wirelesshave made rapid progress in recent years. Recently there has been aconvergence of voice and data networks which is due in part to USderegulation of the telecommunications industry. In order to staycompetitive, companies offering broadband access technologies need tosupport voice, video, and other high-bandwidth applications over theirlocal access networks. For networks that use a shared access medium tocommunicate between subscribers and the service provider (e.g., cablenetworks, wireless networks, etc.), providing reliable high-qualityvoice/video communication over such networks is not an easy task.

A cable modem network or “cable plant” employs cable modems, which arean improvement of conventional PC data modems and provide high speedconnectivity. Cable modems are therefore instrumental in transformingthe cable system into a full service provider of video, voice and datatelecommunications services. Digital data on upstream and downstreamchannels of the cable network is carried over radio frequency (“RF”)carrier signals. Cable modems convert digital data to a modulated RFsignal for upstream transmission and convert a downstream RF signal todigital form. The conversion is done at a subscriber's home. At a cablemodem termination system (“CMTS”) located at a head end of the cablenetwork, the conversions are reversed. The CMTS converts downstreamdigital data to a modulated RF signal, which is carried over the fiberand coaxial lines to the subscriber premises. The cable modem thendemodulates the RF signal and feeds the digital data to a computer. Onthe return path, the digital data is fed to the cable modem (from anassociated PC for example), which converts it to a modulated RF signal.Once the CMTS receives the upstream RF signal, it demodulates it andtransmits the digital data to an external source.

FIG. 1 is a block diagram of a typical two-way hybrid fiber-coaxial(HFC) cable network system. It shows a head end 102 (essentially adistribution hub) which can typically service about 40,000 homes. Headend 102 contains a CMTS 104 that is needed when transmitting andreceiving data using cable modems. Primary functions of the CMTS include(1) receiving signals from external sources 100 and converting theformat of those signals, e.g., microwave signals to electrical signalssuitable for transmission over the cable system; (2) providingappropriate Media Access Control (MAC) level packet headers for datareceived by the cable system, and (3) modulating and demodulating thedata to and from the cable system.

Head end 102 (and CMTS 104) connects through pairs of fiber optic lines106 (one line for each direction) to a series of fiber nodes 108. Eachhead end can support normally up to 80 fiber nodes. Pre-HFC cablesystems used coaxial cables and conventional distribution nodes. Since asingle coaxial cable was capable of transmitting data in bothdirections, one coaxial cable ran between the head end and eachdistribution node. In addition, because cable modems were not used, thehead end of pre-HFC cable systems did not contain a CMTS. Returning toFIG. 1, each of the fiber nodes 108 is connected by a coaxial cable 110to two-way amplifiers or duplex filters 112, which permit certainfrequencies to go in one direction and other frequencies to go in theopposite direction (different frequency ranges are used for upstream anddownstream paths). Each fiber node 108 can normally service up to 500subscribers. Fiber node 108, coaxial cable 110, two-way amplifiers 112,plus distribution amplifiers 114 along with trunk line 116, andsubscriber taps, i.e. branch lines 118, make up the coaxial distributionsystem of an HFC system. Subscriber tap 118 is connected to a cablemodem 120. Cable modem 120 is, in turn, connected to a subscribercomputer 122.

According to a current standard for transmission of data over cablenetworks (termed “DOCSIS”), there is no provision for any redundancy atthe CMTS of the cable system. Therefore, a failure of the CMTS willresult in a service disruption or service outage of the cable modemsrelying upon the failed element. If a CMTS fails, for example, it mayhave to be repaired or replaced before service can resume. This meansthat service can be out for an extended period. From the perspective ofthe service provider and the end user, any type of disruption or delayin service is extremely undesirable.

This problem becomes particularly acute as broadband accesstechnologies, including cable, move toward digital telephony (e.g.,Voice over IP or “VoIP”). For these applications, rapid reliable cutoverfrom a failed component becomes critical. If such technologies are tocompete with analog telephony, a greatly improved protection/cutovertechnology is necessary.

SUMMARY OF THE INVENTION

To address these issues, the present invention provides a redundancytechnique in a shared-access computer network to reduce delaysexperienced by various elements within the network which may be causedby equipment failure, software failure, or other network problems. Theinvention provides a protection CMTS available to immediately service acable modem should that modem's service from a working CMTS fail for anyreason. To speed the service transfer (cutover) from the working CMTS tothe protection CMTS, the cable modem may preregister with the protectionCMTS well before the cutover becomes necessary. The cable modem'sregistration with both the working CMTS and the protection CMTSpreferably employs a single IP address, so that the cable modem need notobtain a new IP address during cutover. Further, to prevent routingconflicts, the working CMTS and the protection CMTS should be designedor configured so that only the working CMTS injects a host route for thecable modem into the appropriate routing protocol. Only after cutover tothe protection CMTS should the protection CMTS inject its host route. Byemploying a redundancy system as described, the cable system can providetelephony service with fewer significant disruptions.

One aspect of the invention provides a method implemented on a workingCMTS for providing redundancy to a cable network. In the cable network,the working CMTS provides normal service to a cable modem and aprotection CMTS takes over service to the cable modem should servicefrom the working CMTS fail. The method may be characterized by thefollowing sequence: (a) participating in registration of the cable modemto allow servicing of the cable modem by the working CMTS and (b)obtaining an IP address for the cable modem, which IP address isobtained from an address space outside the subnet of the working CMTS.The same IP address should used in communications with both the workingCMTS and the protection CMTS. Preferably, the working CMTS and theprotection CMTS are routing CMTSs. Also, preferably the address spacefrom which the IP address for the cable modem is obtained lies outsideof the subnet for the protection CMTS. In other words, the cable modemIP address is not bound to either the working or protection CMTSinterface.

By participating in registration of the cable modem, the working CMTSmay perform certain registration functions, In the case of a DOCSISregistration, for example, the CMTS may specify such parameters as atransmission power, a transmission frequency, and transmission timeslots at which the cable modem is to communicate with the working CMTS.By participating in registration, the working CMTS may also participatein a process to assign the IP address for the cable modem. Such processmay be DHCP and the working CMTS obtains the IP address while performinga DHCP relay function.

In order to allow the protection CMTS to take over service to the cablemodem without requiring assignment of a new IP address, the IP addressused with the working CMTS must be communicated to the protection CMTS.In one embodiment, this is accomplished by a communication from eitherthe cable modem or working CMTS.

In accordance with this invention, the working CMTS may be expected toinject a host route to the cable modem into a routing protocol. In someheadend topologies of this invention, injecting the host route comprisesproviding the host route to one or more aggregation routers servicingthe cable network.

To effect a smooth cutover to the protection CMTS, the working CMTS mayalso perform the following functions: (c) determining that the workingCMTS's service to the cable modem has failed; and (d) notifying at leastone of the cable modem and the protection CMTS that the cable modemshould obtain service from the protection CMTS. This cutover procedureis particularly useful when the service provided to the cable modemincludes telephony service.

Another aspect of the invention provides a CMTS designed or configuredto act as a working CMTS for a cable network including the working CMTSand a protection CMTS. The CMTS may be characterized by the followingfeatures: (a) one or more processors; (b) memory in communication withat least one of the one or more processors; and (c) registration datafor the cable modem. At least one of the processors is configured tostore the registration data in memory. The registration data specifiesan IP address for the cable modem, which IP address resides in anaddress space outside the subnet of the working CMTS. Further, the IPaddress is used for communication with both the working CMTS and theprotection CMTS. In addition, the registration data may include suchinformation as a transmission power, transmission time slots, and/or atransmission frequency at which the cable modem is to communicate withthe working CMTS. Preferably, the CMTS is a routing CMTS that isconfigured to implement DOCSIS.

The CMTS should be designed or configured to inject a host route for thecable modem into a routing protocol. It should also be designed orconfigured to notify at least one of the cable modem and the protectionCMTS that the cable modem should obtain service from the protection CMTSwhen the working CMTS's service to the cable modem has failed.

Yet another aspect of the invention provides a method implemented on aprotection CMTS for providing redundancy for a cable network having aworking CMTS and the protection CMTS. The method may be characterized bythe following sequence: (a) determining that the working CMTS's serviceto the cable modem has failed; and (b) taking over service to the cablemodem while using an IP address for the cable modem that was also usedwhile the working CMTS was servicing the cable modem. As with some ofthe embodiments discussed above, the IP address for the cable modempreferably resides in an address space located outside the subnet forthe working CMTS and also outside the subnet for the protection CMTS.

To determine that the working CMTS's service to the cable modem hasfailed, the protection CMTS may receive a notification to that effectfrom at least one of the cable modem and the working CMTS. To take overservice to the cable modem, the protection CMTS may request that thecable modem adjust at least one of the following parameters:transmission power, transmission frequency, and transmission time slots.

After taking over service to the cable modem, the protection CMTS mayinject a host route to the cable modem, through the protection CMTS,into a routing protocol. This may involve providing the host route toone or more aggregation routers servicing the cable network.

Another aspect of the invention pertains to computer program productsincluding a machine readable medium on which is stored programinstructions for implementing the methods as described above. Any of themethods of this invention may be represented as program instructionsthat can be provided on such computer readable media.

These and other features and advantages of the invention will bepresented below with reference to the associated drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram depicting some of the principal components ofa cable network that may be used with the present invention.

FIG. 2A is a block diagram depicting a cutover procedure using a 1:1topology in accordance with an embodiment of this invention.

FIG. 2B is a block diagram depicting a cutover procedure using a 1:1sparing topology in accordance with an embodiment of this invention.

FIG. 2C is a block diagram depicting a cutover procedure employing a 1:Ntopology in accordance with an embodiment of this invention.

FIG. 2D is a detailed block diagram of a cable network head-endimplementing a 1:1 redundancy topology in accordance with an embodimentof this invention.

FIG. 2E is a detailed block diagram of an alternative head-end topologyfor 1:1 redundancy in accordance with this invention.

FIG. 3A is a process flow diagram depicting some operations employedwithin a cable network during registration of a cable modem inaccordance with one embodiment of this invention.

FIG. 3B is an interaction diagram depicting the interactions of a cablemodem, a working CMTS, and a provisioning server during registration ofthe cable modem in accordance with one embodiment of this invention.

FIG. 4 is a schematic diagram of a cable network illustratingregistration of a cable modem with a working CMTS in accordance with anembodiment of this invention.

FIG. 5 illustrates a cutover procedure resulting from a failure on thepath between the cable modem and the working CMTS.

FIG. 6 is a process flow diagram depicting some operations performed ona cable network during cutover in accordance with an embodiment of thisinvention.

FIG. 7 is an interaction diagram depicting the interactions of a cablemodem, a working CMTS, and a protection CMTS during cutover inaccordance with an embodiment of this invention.

FIG. 8A is a block diagram depicting a CMTS structure that may beemployed with the present invention.

FIG. 8B is a block diagram depicting a cable modem structure that may beemployed with the present invention.

FIG. 9 is a schematic illustration of a wireless network suitable forimplementing the present invention.

DETAILED DESCRIPTION THE PREFERRED EMBODIMENT A. Topologies Examples

FIGS. 2A-2E present various cable network topologies that may be used inimplementing the present invention. FIG. 2A depicts a network topologydeemed “1:1” in which the network includes two CMTSs. Both are workingCMTSs and both provide protection for the other. Thus, if one of the twoCMTSs fails, the other one assumes the functions of the failed CMTS,while maintaining its own functions.

As shown in FIG. 2A, a cable network system 201 includes first andsecond cable modems 203 and 205. Each connects to a separate CMTS.Specifically, modem 203 connects to a CMTS 207 via a downstream channel56 and modem 205 connects to a CMTS 209 via a downstream channel 57.Each connection is made through an HFC network 211. Communicationsbetween the cable modems and external sources are made via a connection200.

Note that FIG. 2A is greatly simplified. Normally, a given CMTS or CMTSinterface services many cable modems. For example, a single CMTS mayhandle one or more distribution networks within a cable plant. Thus,cable modems 203 and 205 may represent groups of modems or an entiredistribution network having numerous cable modems.

CMTS 207 is given the designation “W1” for working group 1. This meansthat it is responsible for handling communications with modem 203 andits peers. Similarly, CMTS 209 is designated “W2,” as it serves needs ofcable modem 205 and possibly many other modems. In accordance with thisinvention, the CMTSs serve additional roles. CMTS 207 provides aprotection path for CMTS 209, while CMTS 209 provides a protection pathfor CMTS 207. Thus, if CMTS 207 fails or otherwise goes out of service,CMTS 209 will take over responsibility for servicing cable modem 203 andits peers. Likewise, if CMTS 209 fails, CMTS 207 will take overresponsibility for cable modem 205 and its peers. Note that thisinvention is not limited to cases in which a working CMTS “fails.” It isalso useful for cases where the user simply wants the modems to move tothe protection CMTS while the user upgrades or services the working CMTSsoftware, hardware, etc.

FIG. 2A illustrates the failure of CMTS 207. As shown, cable modem 203can no longer communicate via CMTS 207 and therefore communicatesthrough CMTS 209. To accomplish this, communications to and from cablemodem 203 take a different path through HFC network 211. Further, cablemodem 203 must shift from downstream channel 56 to channel 57, thechannel of CMTS 209. Thus, in this example, cable modem 203 will tune toa different downstream frequency.

In another topology, deemed “1 for 1 sparing,” the network uses twoCMTSs: one is a normal working CMTS intended to carry on the normallyworking functions of a CMTS and another is dedicated to providingprotection. In this topology, the protection CMTS does not provideservice until the working CMTS fails. It then takes over that machine'sfunctions. FIG. 2B depicts a 1 for 1 sparing topology. As shown, a cablenetwork 201′ includes a working CMTS 213 and a protection CMTS 215.Working CMTS provides service to cable modem 203 and cable modem 205,both over channel 56. This is depicted by the connection paths throughHFC plant 211. Note that protection CMTS 215 does not normally provideservice to any cable modems. It remains available to take over in thecase of a failure.

Assume now that working CMTS 213 fails for some reason. Then, cablemodems 203 and 205 cannot communicate through it. In the embodiment ofFIG. 2B, CMTS 215 takes over the role of CMTS 213. Preferably, thecutover takes place rapidly. It may be necessary for the cable modems toswitch from channel 56 to channel 57 during the cutover, as shown.

In another embodiment, “1 for N sparing,” multiple working CMTSs areprotected by a single protection CMTS. The protection CMTS does notprovide cable service until one of the N working CMTSs fails. Thisnetwork topology is depicted in FIG. 2C. As shown, a network 201″includes three working CMTSs: a CMTS 213, a CMTS 217, and a CMTS 219.CMTS 213 provides service to cable modem 203 over channel 56, CMTS 217provides service to cable modem 205 over channel 57, and CMTS 219provides service to a cable modem 222 over channel 58. A protection CMTS221 does not normally service any cable modems but is available toservice any cable modem in case it needs to take over for a failed peer.Note that in the depicted topology, protection CMTS 221 is assigneddownstream channel 59.

As shown in FIG. 2C, when one of the working CMTSs fails (CMTS 217 inthis instance), protection CMTS 221 takes over its role. Here cablemodem 205 must begin communicating through protection CMTS 221 overchannel 59. Note that the service to cable modems 203 and 222 is notaffected. If working CMTS 213 were to fail, protection CMTS 221 wouldhave to take over for it as well. The same is true for working CMTS 219.

In yet another topology, deemed “1:N” service, the cable networkincludes N+1 working CMTSs, and at least one of these working machinescan provide protection for some or all of the other N machines. Theseapproaches have the benefit of making use of all resources during normaloperation. That is, the protection CMTS does not sit idle as it must inthe “sparing” embodiments. Normally it provides a working path for someof the network modems. However, when a protection/working CMTS isfilling in for a failed CMTS, it may have a rather heavy load.

This invention may employ multiple distinct CMTSs to provide redundancyas discussed in much of the discussion herein. Alternatively, a singleCMTS may provide both working and protection services. In thisalternative embodiment, separate line cards (or more generallyinterfaces) may provide the various functions. Depending upon thenetwork topology, one or more CMTS interfaces may provide the cutoverprotection and one or more interfaces may provide normal workingservice. In one embodiment, if one interface fails another one on thesame CMTS can take over for it.

FIGS. 2D and 2E present detailed examples of head-end topologiesemploying a 1:1 service. The invention is by no means limited to thesetopologies. As shown in FIG. 2D, the cable network head-end 230 includesa first CMTS interface 232 and a second CMTS interface 234. These CMTSinterfaces may be provided on a single CMTS chassis or on separateCMTSs. In this specific embodiment, each interface has one downstreamport, labeled “DS,” and six upstream ports labeled “U0”-“U5.” Downstreamsignals from CMTS 232 are provided at an intermediate frequency. Whenthe signal reaches an upconverter 236, its frequency is increased to alevel associated with cable channel 64.

Signals passing downstream from upconverter 236 encounter a splitter 238which directs them to either a first downstream fiber node 240 or asecond downstream fiber node 242. During normal operation, CMTSinterface 232 services only those cable modems connected through fibernode 240. Should CMTS interface 234 (which normally services fiber 20node 242) fail, however, CMTS 232 can take over service to the cablemodems serviced via fiber node 242.

As shown, CMTS interface 234 provides intermediate frequency downstreamsignals to an upconverter 244. In the example shown, upconverter 244converts the intermediate frequency signal to an RF frequency signalcorresponding to cable channel 65. That downstream signal encounters asplitter 246, which allows the downstream signal to be provided toeither fiber node 240, fiber node 242, or both. During normal operation,CMTS 234 services only those cable modems connected through fiber node242.

Considering now the upstream signal, cable modems provide data on aspecified upstream frequency band to fiber nodes 248 and 250. Normally,upstream data passing through fiber node 248 passes to CMTS interface232 (via port “U0”). If the upstream path to CMTS interface 232 isdisrupted for any reason (e.g., CMTS interface 232 fails), that upstreamdata is provided to CMTS interface 234. To this end, a splitter 252allows data from fiber node 248 to pass through to either interface 232or interface 234. Similarly, a splitter 254 allows upstream date fromfiber node 250 to pass to either of interfaces 232 or 234.

For telephony applications, different cable modems communicating througha given fiber node may transmit at different frequency bands. Thus,different upstream ports on a CMTS interface may be configured to handledifferent ones of these upstream frequency bands. This embodiment isillustrated in topology 230 by the use of upstream splitters 256 and258. Upstream data passing through fiber node 250 may be carried on oneof two possible frequency bands. One of these bands is handled by portU1 on interfaces 232 and 234. The other of these frequency bands ishandled by ports U2 of the interfaces.

Note that the head-end topology depicted in FIG. 2D is intended toprovide full service to the cable network. Thus, a local feed 260provides cable TV service to subscribers via fiber node 240. Similarly,a local feed 262 provides cable TV service to subscribers via fiber node242.

FIG. 2E depicts a slightly different head-end topology (264), whichaccomplishes essentially the same results. In this Figure, networkelements that provide identical function to those depicted in FIG. 2Dare given like reference numbers. As shown, the upstream service, withassociated redundancy, is identical to that depicted in topology 230 ofFIG. 2D.

The downstream network topology is somewhat different, however. In thiscase, each interface is capable of providing downstream data at eitherchannel 64 or channel 65 (in the specific example). As shown, CMTSinterface 232 provides downstream data (on an intermediate frequency) toa splitter 266. During normal operation, splitter 266 directs all datato an upconverter 268, which puts the data on a carrier frequencycorresponding to cable channel 64. This data is then provided todownstream fiber node 240, and then on to destination cable modems. Ifthe downstream path from CMTS interface 234 should fail for any reason,interface 232 takes over responsibility for providing downstream data tothose cable modems normally serviced by interface 234. It accomplishesthis by providing downstream data to an upconverter to 270 (via splitter266). Note that upconverter 270 puts the data on a carrier frequencycorresponding to cable channel 65. That data is then directed to fibernode 242 and then on to the destination cable modems.

CMTS interface 234 provides a backup to interface 232, as well. Asshown, downstream data passes from interface 234 to a splitter 272.During normal operation, splitter 272 directs all downstream trafficthrough an upconverter 274 which puts the data on a carriercorresponding to cable channel 64. This data is then provided to fibernode 242. If interface 234 should be called upon to cover for interface232, splitter 272 will direct the appropriate traffic to an upconverter276, which puts that data on a carrier frequency corresponding to cablechannel 65. This data is then provided to downstream fiber node 240.

B. Two Stages of Cutover

Typically, the protection afforded by this invention affects normalnetwork operation at two stages. In a first stage, the protection CMTS(or interface) is designated for a particular working CMTS (orinterface). In the most trivial case, this simply involves providinginstructions for directing cable modems to the protection CMTS whentheir working paths fail. Other procedures may include a modifiedregistration process, in which the cable modem pre-registers with theprotection CMTS. As explained, the cable modem may obtain a networklevel address (e.g., an IP address) that is not part of the working CMTSinterface subnet (or of the protection CMTS interface subnet). Also, anetwork level routing protocol may be affected during this first stageto limit propagation of the host route through the working CMTS.

In a second stage, failure has occurred and cutover from the working tothe protection device is required. Here the affected cable modemregisters with the protection CMTS, possibly without requiring a newnetwork level address. The protection CMTS may also then begin toadvertise the new host route to the cable modem.

1. Stage 1—Establishing a Cutover Path

Typically, when a cable modem comes on line, it registers with the CMTSthat will serve it. It is possible, in accordance with this invention,that a cable modem that has had its CMTS (or path to that CMTS) failsimply registers with a designated protection CMTS. Unfortunately, mostcable modem registration protocols require that the CM obtain an IPaddress specific to its CMTS. This results because the addressing modelassigns the cable modem an IP address that is part of the IP subnet ofan associated interface (on the working CMTS).

If the CM must use the conventional registration process to registerwith its protection CMTS after a failure on its working path, then itmust obtain an IP address from the protection CMTS's subnet. As part ofthis process, a PC or other machine behind the cable modem may have toreboot. Thus, service may be disrupted for a somewhat lengthy period oftime. This may be unacceptable for some applications, where rapidcutover is required. Further, the cable modem may have had many previousconnections with external nodes using its previous IP address, whichexternal nodes are not immediately aware of the IP address change.Regardless of this issue, DNS and/or a Call Agent will have to getinvolved. Note that a Call Agent is used to maintain a list of client IPaddresses for use in setting up IP telephony calls.

One approach to speeding up the cutover process involves using cablemodem IP addresses that are not part of any particular CMTS's interfacesubnet. Preferably, a registering cable modem obtains its IP addressfrom an address block that is not part of a CMTS interface IP subnet,but is typically on an IP “supernet” shared among various CMTSs. Thenwhen a cutover is required, the cable modem need not obtain a new IPaddress from a different address space. Various protocols may be used toassign the CMTS-independent IP addresses. In one embodiment, aregistering cable modem obtains its IP address from a Dynamic HostConfiguration Protocol (DHCP) server configured to provide IP addressesfrom outside the address space of any CMTS interface. DHCP is describedin RFC 2131, incorporated herein by reference for all purposes.Generally, in this protocol, the computer is told to ask thenetwork—according to prescribed rules—for a temporary network address.

This procedure has the benefit of allowing a cable modems to cutoverfrom a failed path to a protection channel with minimal overhead. As thecable modem is already registered on the protection channel, it need notobtain a new IP address and go through the attendant time-consumingregistration process. Hence service disruption is minimized. The timespent out of service is greatly reduced, connections and context are notnecessarily lost, the host machine need not reboot, etc. Without thesebenefits, applications such as cable telephony may not be realized.

FIGS. 3A, 3B, and 4 illustrate one set of procedures for registering acable modem in accordance with an embodiment of this invention.Referring first to FIG. 3A, a flow chart is presented depictinggenerally the steps that a cable modem (and associated CMTSs) may gothrough to register on both the working CMTS a protection CMTS. Asillustrated, a process 301 begins at 303 with the cable modem submittinga registration request to a working CMTS. In a specific embodiment, theregistration complies with the procedures required by the DOCSISstandard. Normally, this involves obtaining an IP address for the cablemodem, obtaining “ranging” parameters such as upstream frequency, powerand timing, etc.

Next, at 305, the cable modem is assigned an IP address suitable for usewith this invention. In this embodiment, an IP address is chosen so thatthat IP address can be used with both the working CMTS and theprotection CMTS. Thus, the IP address should be chosen from an addressblock that is not dedicated to either the working CMTS or the protectionCMTS. As explained above, under current practice a cable network assignsIP addresses from an address block bound to a particular CMTS interface.Unfortunately, if that interface fails (or the path to it fails) thenthe IP address that has been assigned to the cable modem is no longeruseful. As a consequence, the cable modem must obtain a different IPaddress if it is to communicate through a protection CMTS. To avoid thisproblem, this embodiment of the present invention requires that the IPaddress that has been assigned to the cable modem during registration beselected from the address space lying outside the address blocksassigned to either the working CMTS or protection CMTS.

Because the working CMTS participates in the registration process, itcan determine the IP address that has been assigned to the cable modem.This is illustrated at 307 where the working CMTS notes the assignedcable modem IP address and injects the associated host route into theappropriate routing protocol. The host route, in this instance,specifies the route to the registering cable modem through the workingCMTS.

The host route is preferably provided to one or more aggregation routersassociated with the head-end of the cable network. This is depicted at309 in process 301. Because the host route specifies the working CMTS,and provision is made for having a protection CMTS take over for theworking CMTS, the host route should not propagate beyond the head-end.Then, when the working CMTS fails and the protection CMTS takes over,the new host route can quickly replace the previous host route in therelevant routers.

Next, the cable modem obtains the relevant registration parameters,including its IP address, and is also informed of the protection RFchannel. See 311. Note that the normal registration parameters includean upstream transmission frequency, an upstream transmission power, timeslots for upstream transmission, etc.

Because the protection CMTS communicates via a different upstream RFchannel than the working CMTS, it is necessary to inform the cable modemof the protection CMTS's upstream channel. With the contact informationin hand, the cable modem re-registers on the protection channel with theprotection CMTS. See 313. The cable modem will obtain the registrationparameters for the protection CMTS and store them in preparation for anevent that causes it to cutover. Note that this re-registration processdoes not assign a new IP address to the cable modem. Rather, the cablemodem preserves the IP address that was assigned to it duringregistration on the working CMTS.

Finally, at 315, the protection CMTS recognizes the cable modem, butdoes not inject a host route for that cable modem into the routingprotocol. If the upstream path to the working CMTS fails, and the switchover to the protection CMTS is required, the protection CMTS willrapidly accept the pre-registered cable modem. Until that time, however,the protection CMTS does not advertise its host route to the cablemodem.

After pre-registration, but before cutover, the protection CMTS remainsin a “protection state” ready to take over service to the cable modemwhen it determines that the modem's working route has failed. While inthe protection state, the protection CMTS may periodically ensure thatit is ready to take over service to the cable modem. This may entailthat the protection CMTS determine that the protection path still works.If communication can take place over the path, the protection CMTS mayrequest that the cable modem change certain parameters to optimizecommunication if a cutover becomes necessary. As indicated, thetransmission characteristics of a cable network path vary withtemperature, load, mechanical conditions, etc. Thus, what were optimaltransmission settings one day, may be far from optimal the next day.

If the cable network uses DOCSIS, the protection CMTS may periodicallyissue station maintenance opportunities to the cable modem. In response,the cable modem sends a ranging request message at a transmission powerand frequency as specified by its stored parameters. The protection CMTSdetects the power, frequency, and timing of the ranging request. Itdetermines how far these parameters vary from optimal, if at all, andsends a ranging response message instruction the cable modem to changeits parameters as necessary. The protection CMTS may also use a DOCSISping to determine whether the protection path works.

FIG. 3B presents an interaction diagram for cable network componentsused in a specific embodiment of the present invention. Again, thisembodiment involves registration of a cable modem in a manner allowingrapid cut over to a protection CMTS if a working CMTS fails. Asillustrated in FIG. 3B, the relevant components are a cable modem 321, aworking CMTS 323, and a provisioning server 325.

Initially, a new cable modem comes on line at 320. It then sends aregistration request (322) to working CMTS 323. As part of theregistration procedure, working CMTS 323 requests an IP address for thecable modem from provisioning server 325. See arrow 324. In a preferredembodiment, provisioning server 325 is running DHCP, which allows it toassign an IP address to cable modem 321 as indicated by operation 326.Subsequently, provisioning server 325 forwards the IP address to workingCMTS 323. See arrow 328.

Working CMTS 323 now has all the information it requires to completeregistration of cable modem 321. As part of the registration process, itrecords the assigned IP address of cable modem 321. See operation 330.Working CMTS 323 then forwards the IP address and registrationinformation to cable modem 321 as indicated by arrow 332. Concurrently,working CMTS 323 injects the host route for the cable modem into therelevant routing protocol. See operation 334.

Now that cable modem 321 is registered on the working CMTS, the cablenetwork can begin pre-registering the cable modem on the protectionCMTS. In the specific embodiment depicted in FIG. 3B, thispre-registration process begins with provisioning server 325 informingcable modem 321 of the protection CMTS. As indicated in the discussionof FIG. 3A, this may involve informing the cable modem of the radiofrequency channel for the protection CMTS. Regardless of the specifics,the operation of informing the cable modem is depicted by arrow 336 inFIG. 3B. After it has been informed in this manner, cable modem 321initiates the registration procedure with the protection CMTS in amanner such as that described with reference to FIG. 3A.

In this example, provisioning server 325 serves various functions. Itmay normally be used to provide various telephony support services forVoIP. Server 325 may run on an arbitrary piece of hardware such as a Sunworkstation or other Unix system, a Windows NT server and the like. Inthe depicted embodiment, the provisioning server implements DHCP as wellas other relevant functions for the cable network. For example, it maycontain a list of MAC addresses for cable modems associated with variouspaying customers. Associated with this list is the type of serviceavailable to each cable modem. For example, those subscribers havingtelephony service will be identified. When a cable modem registers, theprovisioning server will recognize that it is a telephony subscriber andtherefore cause it to register on both the working and protection CMTSs.

As mentioned, associated with the registration process, the cablenetwork head-end injects the relevant host route into an appropriaterouting protocol. FIG. 4 illustrates this process schematically.Normally, a CMTS advertises routes to its cable modems by identifyingits interface subnet(s) via the appropriate routing protocol. In anembodiment of this invention, the CMTS advertises only a very smallchunk of address space, not normally associated with its interfaces.These addresses provide host routes or small chunks of address spaceincluding IP addresses assigned to the cable modems during registration.Note that when the advertised address space is so small as to identifyonly a single cable modem, that “chunk” of address space, as used in arouting protocol, is referred to as a “host route.”

Various routing protocols are in use. These include OSPF, RIP, and IGRP.In general, these protocols allow routers to exchange informationidentifying chunks of IP address space that they know about and/or areservicing. Conventionally, as part of a routing protocol, a CMTS may letits peer routers know that it handles an address space given by thesubnet/255.255.255.0, for example. In other words, all cable modems thatthe CMTS handles have IP addresses falling within this address mask.Because the CMTS provides this address mask to its peers via a routingprotocol, they know that if they have a packet destined for a nodehaving IP address within the subnet, they should transmit the packet tothe CMTS. In this invention, the CMTSs advertise specific host routesalone or in addition to their specific interface subnets.

As shown in FIG. 4, an HFC network 400 supports various cable modems,including cable modems 401, 403, 405 and 407. Each of these cable modemsmay be serviced by a separate fiber node, for example. In the networksituation depicted, cable modem 401 has just registered and is obtainingits IP address from a provisioning server 409. If provisioning server409 is employing DHCP to assign IP addresses, a working CMTS 411 willserve the DHCP relay function. By performing this function, CMTS 411gleans the IP address that has been assigned to cable modem 401. Itrecords this information. Of course, other procedures may be employed toassign IP addresses to cable modems coming on line. Preferably suchprocedure should allow for notifying working CMTS 411 of newly assignedIP addresses for its cable modems.

As shown, the head-end of the cable network includes multipleaggregation routers. These routers serve to facilitate communicationbetween the cable network and external sources. In the specificembodiment shown in FIG. 4, there are two aggregation routers, a router413 and a router 415. After CMTS determines the IP address of newlyregistered cable modem 401, it injects the host route for that cablemodem into the routing protocols used by aggregation routers 413 and415, as illustrated. This host route specifies that cable modem 401 canbe reached through CMTS 411. Aggregation routers 413 and 415 areconfigured to limit propagation of this host route to routers within thehead-end. In a preferred embodiment, the working and protection CMTSs ofthis invention are routing CMTSs, and therefore participate in thenecessary routing procedures. One example of the hardware and softwareemployed in such routing CMTSs is described below in connection with thedescription of FIG. 8A.

In the topology depicted in FIG. 4, multiple CMTSs on a cable plantconnect to external networks via one or more higher-level aggregationrouters (routers 413 and 415 in this example). Each CMTS on the cablenetwork is responsible for its own group of cable modems with associatedhost routes and address/mask. Each of these CMTS advertises its portionof IP address space to the higher-level router(s). A higher-level routerin possession of this information, then advertises to its peers via arouting protocol that it can handle packets having destination addressesfalling within any of the host routes and address blocks of theunderlying CMTSs.

Note that provisioning server 409 is connected to HFC network 400. Inthe embodiment described with reference to FIGS. 3A and 3B, provisioningserver 409 informs cable modem 401 of a protection CMTS. In theembodiment depicted in FIG. 4, a CMTS 417 serves as the protection CMTSfor cable modem 401. As illustrated, CMTS 417 can communicate withprovisioning server 409 and thereby use its services.

In the above-described embodiments, some technique is required fornotifying the protection CMTS of the cable modem's IP address. There areat least three preferred approaches to informing the protection CMTS.The first requires that the cable modem notify the protection CMTS ofthe IP address that it has obtained during an initial registrationthrough the working CMTS. This notification may serve as a part of theDOCSIS registration process with the working CMTS. In this embodiment,the cable modem may or may not complete a complete registration (rangingand the like) with the protection CMTS. Regardless of the level ofpre-registration, the protection CMTS can automatically advertise thehost route to the cable modem, should the working path fail.

In a second approach, the cable modem separately registers through theprotection CMTS (before or after it registers through the working CMTS)and obtains an IP address during registration. This IP address isidentical to the IP address that the cable modem obtains via the workingCMTS. Because the protection CMTS participates in the registrationprocess, it records the cable modem's IP address. If DHCP is used, thenthe DHCP server will recognize that the cable modem requesting an IPaddress has already obtained such address and will merely assign thesame address during the second registration. A third approach requiresthat the working CMTS communicate the cable modem's IP address to theprotection CMTS. This may be accomplished via a special protocol forcommunication between the working and protection CMTSs.

2. Stage 2—The Cutover Process

FIG. 5 schematically presents the working and protection paths thatcable modem 401 may employ. As shown, cable modem 401 normallycommunicates through CMTS 411 using downstream channel 56.Communications to and from external networks are routed throughaggregation router 413. A redundant router 415 is provided to backuprouter 413 should it fail.

If CMTS 411 fails (or some component on the path from CMTS 411 fails),cable modem 401 reconnects through protection CMTS 417. Note that inthis Figure, downstream communication from CMTS 417 is conducted overchannel 57. After CMTS 401 reconnects to the protection CMTS, traffic isrouted through that CMTS and aggregation router 413.

One event that must occur during the cutover is notification, via theappropriate routing protocol(s), that a new working CMTS (the protectionCMTS) now provides access to the cable modems previously handled by afailed CMTS. In the embodiment of FIG. 5, part of the cutover processrequires that protection CMTS 417 inject its host route to cable modem401 into the appropriate routing protocol. Preferably, this is aconventional process such as described above with reference to theworking CMTS.

FIG. 6 presents a process flow diagram illustrating fault recovery stepsin accordance with a specific embodiment of this invention. As shown, aprocess 602 beings at 604 with either the cable modem or the workingCMTS detecting a failure. Various mechanisms for detecting such failureswill be discussed below.

After failure detection, the cable modem loads, at 606, its previouslystored protection path parameters. These include, for example, theupstream and downstream frequency bands for communicating with theprotection CMTS, the appropriate cable modem transmission power to theprotection CMTS, the communications time slots allotted for theprotection CMTS, etc. Then, at 608, the cable modem connects to theprotection CMTS and trims its parameters. Note that signal transmissionproperties vary nearly continually within a cable network. As aconsequence, the parameters obtained during the pre-registration stagemay no longer be optimal for communication with the protection CMTS.Trimming simply refers to the process of reoptimizing the transmissionfrequency, power, timing, etc. in light of current network conditions.In accordance with the DOCSIS protocol, trimming may be accomplished bythe “ranging” process.

After the cable modem confirms that the protection CMTS path is working(via ranging, for example), it announces to the protection CMTS that thepath is in fact working. See block 610. This announcement may takevarious forms; e.g., a maintenance ranging request. Thereafter, at 612,the protection CMTS injects its host route into the routing protocol. Ifthe working CMTS has not already stopped injecting its host route intothe routing protocol, it now stops. The receiving aggregation router (orrouters) aggregates the new host route in a manner that preventspropagation outside of the head-end. See 614. At this point, upstreampackets are immediately successful, and, very soon thereafter afterinterior gateway protocol convergence, external packets now transitthrough the correct CMTS and reach the cable modem.

FIG. 7 provides an interaction diagram depicting the interaction of acable modem 701, a working CMTS 703, and a protection CMTS 705 duringcutover in accordance with a specific embodiment of this invention.Initially, at 707, cable modem 701 detects a failure. Alternatively, at709, the working CMTS detects a failure. Either way, the devicedetecting the failure announces it to the other device. See arrow 711.

After the failure has been detected and announced, cable modem 701 loadsthe protection CMTS parameters as indicated by arrow 713. Using theseparameters, it then attempts to reconnect with the protection CMTS 705.See arrow 715. Reconnection may involve sending a DOCSIS rangingrequest.

Upon receipt of the appropriate connection request from cable modem 701,protection CMTS 705 confirms that the cable modem is transmitting in amanner that allows the CMTS to service it. See arrow 717. In a DOCSISprotocol, this procedure may involve confirming that the frequency,power and timing of a ranging request are appropriate. In any event,protection CMTS 705 replies to cable modem 701 as indicated by arrow719. Following the DOCSIS example, the reply will be a ranging responsethat includes any necessary changes to transmission frequency, power,and/or timing. When in receipt of this information, cable modem 701 cantrim its parameters as appropriate. See 721. Next, cable modem 701announces that it is working as indicated by arrow 723. At this point,the protection CMTS injects the new host route into the routing protocolas indicated at 725.

As emphasized herein, the systems and methods of this invention mayprovide fail over protection when there is a detected failure. Suchfailure may be an equipment failure (e.g., all operations of a CMTScease), a circuit failure (e.g., on line card serving a subsection ofthe cable network fails), extreme noise (significant noise occurs over awide frequency band and/or for an extended period of time), etc. Thefollowing examples illustrate the range of possible failures. In onecase, a downstream circuit on a line card fails but upstream circuitcontinues to function. Even though the upstream route still functions,it may be most efficient to have the protection CMTS take over bothupstream and downstream service. In another example, a fiber noderesiding between a working CMTS and its cable modems fails. While theworking CMTS is still operational, the path to it is not. In this case,the correction would require that upstream and downstream data bypassthe inoperative fiber node. This likely means that the working CMTS cannot be used until the fiber node is repaired or replaced. A protectionCMTS is then employed.

In normal operation (according to a standard such as DOCSIS), thereshould be continual “chit chat” between the CMTS and its modems. Thesemessages are often sent at the link or MAC level. In DOCSIS, themessages take the form of pings and/or ranging requests. These messages,which are sent at least about every 30 seconds, confirm that theupstream and downstream paths between cable modem and CMTS areoperational. If the CMTS should go down or some part of the path betweenit and the cable should become inoperational, then the cable modem willrecognize that it can no longer communicate. At that point, it may beginthe cutover procedure. In another scenario, the downstream path isoperational, the cable modem is operational, and the CMTS isoperational. The upstream path, however, is inoperational. The CMTS willrecognize that it is not receiving messages from the cable modem. It maythen infer that the upstream path has a problem and initiate the cutoverto its protection CMTS. These examples illustrate that either the headend or the cable modems can initiate the cutover from a working to aprotection path. This capability provides high system reliability.

C. CMTS Configurations

Generally, the techniques of the present invention may be implemented onsoftware and/or hardware. For example, they can be implemented in anoperating system kernel, in a separate user process, in a librarypackage bound into network applications, on a specially constructedmachine, or on a network interface card. In a specific embodiment ofthis invention, the methods of the present invention are implemented insoftware such as an operating system or in an application running on anoperating system.

A software or software/hardware hybrid system of this invention ispreferably implemented on a general-purpose programmable machineselectively activated or reconfigured by a computer program stored inmemory. Such programmable machine may be a network device designed tohandle network traffic. Such network devices typically have multiplenetwork interfaces. One important class of device that may be used toimplement the present invention is the cable modem termination system.Preferably, the CMTS is a “routing” CMTS, which handles at least somerouting functions. Alternatively, the CMTS may be a “bridging” CMTS,which handles only lower-level tasks.

FIG. 8A provides an example of some components of a CMTS that may beused to implement certain aspects of this invention. In the specificembodiment as shown in FIG. 8A, a CMTS 804 provides functions on threenetwork layers including a physical layer 832, a Media Access Control(MAC) layer 830, and a network layer 834. Generally, the physical layeris responsible for receiving and transmitting RF signals on the cableplant. Hardware portions of the physical layer include a downstreammodulator and transmitter 806 and an upstream demodulator and receiver814. The physical layer also includes software 886 for driving thehardware components of the physical layer.

Upstream optical data signals (packets) arriving via an optical fibernode 810 are converted to electrical signals by a receiver 812. Next,the upstream information packet (RF electrical signals) is demodulatedby the demodulator/receiver 814 and then passed to MAC layer block 830.A primary purpose of MAC layer 830 is to encapsulate, with MAC headers,downstream packets and decapsulate, of MAC headers, upstream packets. Inone embodiment, the encapsulation and decapsulation proceed as dictatedby the above-mentioned DOCSIS standard for transmission of data or otherinformation. Note that at the time when this document was filed, theDOCSIS standard was described in the “Data-Over-Cable Service InterfaceSpecifications—Radio Interface Specifications” SP-RFIv1.1-I02-990731,Interim Specification Jul. 31, 1999. That document is incorporatedherein by reference for all purposes. The MAC headers include addressesto specific modems or to a hub (if sent upstream) by a MAC layer block830 in CMTS 804. Note that the cable modems also include MAC addressingcomponents. In the cable modems, these components encapsulate upstreamdata with a header containing the MAC address of the hub.

MAC layer block 830 includes a MAC hardware portion 804 and a MACsoftware portion 884, which together serve the above-describedfunctions. In a preferred embodiment, MAC hardware portion 804 isdistinct from the router's general-purpose microprocessor and isdedicated to performing some MAC layer functions.

After MAC layer block 830 has processed the upstream information, it isthen passed to network layer block 834. Network layer block 834 includesswitching software 882 for causing the upstream information packet to beswitched to an appropriate data network interface on data networkinterface 802. When a packet is received at the data network interface802 from an external source, the switching software within network layer834 passes the packet to MAC layer 830. MAC block 804 then transmitsinformation via a one-way communication medium to downstream modulatorand transmitter 806. Downstream modulator and transmitter 806 takes thedata (or other information) in a packet structure and converts it tomodulated downstream frames, such as MPEG or ATM frames, on thedownstream carrier using, for example, QAM 64 modulation (other methodsof modulation can be used such as CDMA (Code Division Multiple Access)OFDM (Orthogonal Frequency Division Multiplexing), FSK (FREQ ShiftKeying)). The return data is likewise modulated using, for example, QAM16 or QSPK. Data from other services (e.g. television) is added at acombiner 807. An optical converter 808 converts the modulated RFelectrical signals to optical signals that can be received andtransmitted via Fiber Node 810 to the cable modem hub.

Note that alternate embodiments of the CMTS (not shown) may not includenetwork layer 834. In such embodiments, a CMTS device may include only aphysical layer and a MAC layer, which are responsible for modifying apacket according to the appropriate standard for transmission ofinformation over a cable modem network. The network layer 834 of thesealternate embodiments of CMTS devices may be included, for example, aspart of a conventional router for a packet-switched network. In aspecific embodiment, the network layer of the CMTS is configured as acable line card coupled to a standard router that includes the physicallayer block 832 and MAC layer block 830. Using this type ofconfiguration, the CMTS is able to send and/or receive IP packets to andfrom the data network interface 802 using switching software block 882.

The data network interface 802 is an interface component betweenexternal data sources and the cable system. The external data sourcestransmit data to the data network interface 802 via, for example,optical fiber, microwave link, satellite link, or through various media.The data network interface includes hardware and software forinterfacing to various networks such as, for example, Ethernet, ATM,frame relay, etc.

As shown in FIG. 8A, CMTS 804 includes a central hardware block 850including one or more processors 855 and memory 857. These hardwarecomponents interact with software and other hardware portions of thevarious layers within the CMTS. They provide general purpose computingpower for much of the software. Memory 857 may include, for example, I/Omemory (e.g. buffers), program memory, shared memory, etc. Hardwareblock 850 may physically reside with the other CMTS components. In oneembodiment, the software entities 882, 884, and 886 are implemented aspart of a network operating system running on hardware 850. Preferably,the protective registration and cutover functions of this invention areimplemented in software as part of the operating system. In FIG. 8A,such software may be part of MAC layer software 884 and/or the switchingsoftware 882, or may be closely associated therewith. Of course, theregistration and cutover logic could reside in hardware, software, orsome combination of the two.

The procedures employed by the working and protection CMTSs duringregistration and pre-registration are preferably performed at the MAClayer of the CMTS logic. Thus, in CMTS 804, most of the registrationoperations would be performed by the hardware and software provided forMAC layer logic 830. Associated with the registration are adjustments tothe cable modem's transmission power and transmission frequency. Toallow MAC layer logic 830 to implement such adjustments, it may usepower readings (and sometimes frequency and signal to noise ratioreadings) from an amplitude estimator 816 forming part of the physicallayer logic 832.

The operations associated with obtaining an IP address for cable modemsare preferably implemented at the network layer lever 834. As noted,this may involve the CMTS communicating with a DHCP server via datanetwork interface 802, for example. In addition, network layer logic 834is typically responsible for the operations required to inject hostroutes into the appropriate routing protocols.

FIG. 8B presents a block diagram of a cable modem 890 suitable for usewith this invention. As shown, modem 890 contains many logic blocks,hardware elements, and software elements similar to those of CMTS 804. Amemory 857′ should be able to store registration parameters from bothworking and protection CMTSs. Note that rather than keeping track ofinformation for all cable modems serviced by a CMTS interface, modem 890need only keep track of its own parameter (e.g., power, frequency, timeslots . . . ). Thus, the memory 857′ and processors 855′ need not havethe storage and processing capacities of their counterparts in CMTS 804.

As shown, interface 802′ connects to a PC or other node associated withthe cable modem. At the other end of modem 890, a module 806′ modulatesand transmits upstream data and a module 814′ demodulates and receivesdownstream data. The roles of these blocks are reversed at the CMTS,which sits at the other end of the cable network. Further, thedownstream and upstream lines combine directly to a coaxial cable 892.

The redundancy methods of this present invention may be implemented onvarious general purpose cable modem termination systems. In a specificembodiment, the systems of this invention may be specially configuredCMTSs such as, for example, specially configured models in the uBR-7200series of CMTSs available from Cisco Systems, Inc. of San Jose, Calif.In an alternative embodiment, the methods of this invention may beimplemented on a general-purpose network host machine such as a personalcomputer or workstation. Further, the invention may be at leastpartially implemented on a card (e.g., an interface card) for a networkdevice or a general-purpose computing device.

Although the system shown in FIG. 8A represents one specific CMTSarchitecture of the present invention, it is by no means the only CMTSarchitecture on which the present invention can be implemented. Forexample, other types of interfaces and media could also be used with theCMTS.

Regardless of network device's configuration (for cable plants orotherwise), it may employ one or more memories or memory modules (e.g.,memory 857) configured to store program instructions for the networkoperations and other functions of the present invention describedherein. The program instructions may specify an operating system and oneor more applications, for example. Such memory or memories may also beconfigured to store data structures or other specific non-programinformation described herein.

Because such information and program instructions may be employed toimplement the systems/methods described herein, the present inventionrelates to machine-readable media that include program instructions,state information, etc. for performing various operations describedherein. Examples of machine-readable media include, but are not limitedto, magnetic media such as hard disks, floppy disks, and magnetic tape;optical media such as CD-ROM disks; magneto-optical media such asfloptical disks; and hardware devices that are specially configured tostore and perform program instructions, such as read-only memory devices(ROM) and random access memory (RAM). The invention may also be embodiedin a carrier wave travelling over an appropriate medium such asairwaves, optical lines, electric lines, etc. Examples of programinstructions include both machine code, such as produced by a compiler,and files containing higher level code that may be executed by thecomputer using an interpreter.

Presented below is a very specific methodology and message format tohandle redundant CMTSs and a CMTS-cable modem (CM) protocol for quickranging to the backup CMTS and quick cutover when needed. Many of theterms and procedures presented here are described in detail in theDOCSIS standard, version 1.1, previously incorporated by reference.

-   -   Media Access Control Specification    -   MAC Management Messages    -   Downstream Channel Change Request (DCC-REQ)

A DCC-REQ may be transmitted by a CMTS to a CM to switch to thedownstream channel that the CM is using. The format of a DCC-REQ may beas shown in Figure below:

-   -   Transaction ID Unique identifier for this transaction assigned        by the CMTS    -   Action Code The appropriate Action Code; the CM may behave as        follows    -   0=Switch to the Protect downstream channel do initialization    -   1=Switch to the Protect downstream channel do occasional ranging    -   2=Switch to the Protect downstream channel after failure    -   3=Switch to the Working downstream channel    -   All other parameters are coded as TLV tuples.

When the Action Code is 0, 1, or 2, the DCC-REQ message may contain thefollowing TLVs. When Action Code is 3, the DCC-REQ message must containthe following TLVs.

-   -   Downstream Frequency The downstream frequency which the CM is to        switch to.    -   Priority The priority of this backup channel

If the downstream frequency is not explicitly stated with the downstreamfrequency TLV, then the CM must choose a downstream frequency based uponthe list of downsteam frequencies and their priorities that wereprovided during configuration. If the list does not exist, or thefrequencies are not working, the CM must begin searching for a newdownsteam.

When the Action Code is 0, 1, or 3, if the CMTS does not get a DCC-RSPafter T9 timeout, it must retry.

The CMTS should provide each CM an Occasional Ranging opportunity withthe Protect CMTS at least once every 24 hour period.

Downstream Channel Change Response (DCC-RSP)

An DCC-RSP must be transmitted by a CM to a CMTS in response toreceiving a DCC-REQ if the DCC-REQ Action Code is a 0, 1, or 3. IfDCC-REQ Action Code was a 0 or a 1, the CM must send a DCC-RSP after ithas returned back from the Protect CMTS. If the DCC-REQ Action Code wasa 3, the CM must send a DCC-RSP before it returns to the Working CMTS.If the DCC-REQ Action Code was a 2, the CM must not send a DCC-RSP.

The format of a DCC-RSP message may be as shown in Figure below:

-   -   Transaction ID Transaction ID from corresponding DCC-REQ    -   Response 0=Okay        -   1=Failure

All other parameters are coded as TLV tuples.

The DCC-RSP message may contain:

-   -   Downstream Frequency The downstream frequency which the CM is to        switch to.    -   Priority The priority of this backup channel        -   Cable Modem—CMTS Interaction    -   Cable Modem Initialization    -   For Working CMTS, following the standard procedure and:    -   Transfer Operational Parameters

The CM's config. file may contain the Backup Downstream Channel Set TLV.If present, the CM must send them in the Registration Request.

After the CM initializes with the Working CMTS, the Working CMTS shouldsend a DCC-REQ with an Action Code of 0 to allow the CM to initializewith the Protect CMTS. When the CM registers with the Protect CMTS, itfollows the standard procedure with the exception of skipping:

-   -   Establish IP Connecivity    -   Establish Time of Day    -   Transfer Operational Parameters    -   Several TLVs have been added in REG-REQ and REG-RSP.    -   Registration    -   Registration Request must contain the following TLVs:    -   Modem Primary SID for Protect CMTS with Initialization SID 0;    -   Modem IP Address for Protect CMTS with the CM's current IP        address;

The Registration Response must contain the following TLVs:

Modem Primary SID for Protect CMTS with the assigned primary SID ifprovided now or Initialization SID 0 if provided later duringfailure-over.

Modem IP Address for Protect CMTS with the same IP address if it cansupport or Initialization IP address of 0.0.0.0 if CM must invoke DHCPmechanisms to obtain an IP address later when failure.

Modem Occasional Ranging SID for Protect CMTS. The Protect CMTS mustallocate a contention ranging opportunity with a region large enough toaccount for the variation in delays between any two CMs.

Modem Failure Ranging SID for Protect CMTS. The Protect CMTS mustallocate a contention ranging opportunity with region large enough toaccount for the variation in delays between any two CMs.

-   -   Baseline Privacy Initialization

If the CM is provisioned to run Baseline Privacy, the CM must skip itnow, and initialize Baseline Privacy operations later during failureswitch.

-   -   Standard Operation    -   Changing Downstream Channels

The Working CMTS must provide each CM an Occasional Ranging opportunitywith the Protect CMTS at least once every 24 hour period by sending aDCC-REQ with an Action Code equal to 1. If the CMTS does not get DCC-RSPafter T9 timeout, it must retry.

When a CM performs Occasional Ranging with the Protect CMTS, the CM mustsend the RNG-REQ message using the Occasional Ranging SID. If theOccasional Ranging SID is equal to the Initialization SID 0, than the CMmust use the ranging backoff parameter in the current MAP. The ProtectCMTS must allocate a contention ranging opportunity with a region largeenough to account for the variation in delays between any two CM. The CMmust finish Occasional Ranging within T9 timeout.

The CM may accumulate the adjustment of the ranging parameters with theWorking CMTS, and apply it to the ranging parameters for the ProtectCMTS to better estimate the CM's initial ranging parameters whenswitching to the Protect CMTS.

-   -   Changing Upstream Burst Parameters    -   Never change for Protect CMTS    -   Changing Upstream Channels    -   Never change for Protect CMTS    -   Failure Switch Mode

When a CM receives a DCC-REQ with Action Code 2 or when a CM detectsdownstream failure, the CM will Failure Switch using the followingsteps:

-   -   step1: CM switches downstream channels and synchronize with the        Protect downstream channel.    -   step2: For Failure Ranging, the CM must send the RNG-REQ using        the Failure Ranging SID. The Protect CMTS must send the RNG-RSP        message with the Ranging Status=“success” and with the a Primary        SID for use with the Protect CMTS.    -   step3: If the CM IP Address for Protect CMTS is the        Initialization IP address of 0.0.0.0, then the CM must invoke        DHCP mechanisms to obtain an IP address.    -   step4: If the CM is provisioned to run Baseline Privacy, the CM        must initialize Baseline Privacy operations.

The Protect CMTS must allocate contention ranging opportunities with aregion large enough to account for the variation in delays between anytwo CMs.

-   -   Parameters and Constants    -   System Name Time Reference Minimum Value Default Value Maximum        Value    -   CMTS T9 Wait for DCC-RSP 5    -   CMTS DCC-REQ Retries Number of Retries on DCC-REQ 3    -   Common Radio Frequency Interface Encodings    -   Backup Downstream Channel Set: This field defines the parameters        associated with Backup Downstream Channels    -   Type Length Value: 29 n    -   Priority: The priority of this backup channel    -   Type Length Value: 29.1 1 0-7

More than one backup downstream channel may have the same priority. Inthis case, the CM must scan for these channels from lowest to highestfrequency.

Downstream Frequency: The receive frequency to be used by the CM. Thisis the center frequency of the downstream channel in Hz stored as a32-bit binary number. Downstream Frequency is the unique index of theBackup Downstream Channel Set.

-   -   Length Value: 29.2 4 Rx Frequency    -   Valid Range: The receive frequency must be a multiple of 62599        Hz

Downstream In-Active Timer: Timer in msec that the CM uses to detectdownstream failure before it switchs to the Protect CMTS. This timershould be a level timer based on the highest QoS the CM has.

-   -   Type Length Value: 29.4 4 Downstream in-active timer

Modem Primary SID for Protect CMTS: This is a 16-bit field of which thelower 14 bits define the SID with bits 14 and 15 defined to be 0. Duringinitialization with Protect CMTS, the CM must send REG-REQ messagecontaining this TLV with an Initialization SID 0. Protect CMTS must sendREG-RSP message containing this TLV with an assigned Primary SID ifprovided now or Initialization SID of 0 if provided later when a failureoccurs. During failure switchover, the Protect CMTS must send RNG-RSPmessage with Ranging Status=“success”, containing this TLV with theassigned primary SID.

-   -   Type Length Value: 29.5 2 SID

Modem IP Address for Protect CMTS: The IP address of the CM when it isin normal operation with the Protect CMTS. During initialization withthe Protect CMTS, the CM must send REG-REQ message containing this TLVwith its current IP address. The Protect CMTS must send a REG-RSPmessage containing this TLV with the same IP address if it can supportthe address, or the Initialization IP address of 0.0.0.0 if it cannot.If the CM receives 0.0.0.0, the CM must invoke DHCP mechanisms to obtainan IP address when it performs registration on the Protect CMTS.

-   -   Type Length Value: 29.6 4 IP Address

Modem Occasional Ranging SID for Protect CMTS: SID is a 16-bit field ofwhich the lower 14 bits define the SID with bits 14, 15 defined to be 0.During initialization with Protect CMTS, Protect CMTS must send REG-RSPmessage contains this TLV. When CM do Occasional Ranging with ProtectCMTS, CM must send the RNG-REQ use this Occasional Ranging SID, ifOccasional Ranging SID is Initialization SID 0, than use the rangingbackoff in the current MAP. Protect CMTS must allocate contentionranging opportunity with region large enough to account for thevariation in delays between any two CMs, CM must finish OccasionalRanging within T9 timeout.

-   -   Type Length Value: 29.7 4 SID,    -   Occasional Ranging backoff start,    -   Occasional Ranging backoff end

Modem Failure Ranging SID for Protect CMTS: The SID is a 16-bit field ofwhich the lower 14 bits define the SID with bits 14 and 15 defined to be0. During the initialization with Protect CMTS, the Protect CMTS mustsend a REG-RSP message containing this TLV. During failure switch-over,when the CM does Failure Ranging with the Protect CMTS, the CM must sendthe RNG-REQ using this Failure Ranging SID. The Protect CMTS must send aRNG-RSP with Ranging Status=“success” message and containing the ModemPrimary SID for Protect CMTS TLV with the assigned Primary SID. TheProtect CMTS must allocate a contention ranging opportunity with aregion large enough to account for the variation in delays between anytwo CMs.

-   -   Type Length Value: 29.8 4 SID,    -   Failure Ranging backoff start,    -   Failure Ranging backoff end    -   CMTS Redundancy    -   Overview

DOCSIS systems which are intended to be used for high availabilityapplications such as voice or mission critical data need to be able tooffer rundancy in equipment in order to protect from either CMTS failureof HFC plant failure. The general approach that DOCSIS follows is toprovide the CM access to two (or more) CMTS domains, and let the CMswitch-over from the first CMTS, known as the Working CMTS, to thesecond CMTS, known as the Protect CMTS, when the CM determines there isa failure with the Working CMTS.

The issues addressed include:

-   -   What is the criteria for the CM to switch from the Working CMTS        to Protect CMTS?    -   What is the criteria for the CM to switch from the Protect CMTS        to Working CMTS?    -   Allowing the Working and Protect CMTS to support traffic at the        same time.

Allowing the CM to be moved between CMTS domains for the purposes ofload sharing.

Ranging on the Protect CMTS.

Management of Service Flows, SIDs, IP Addresses, and Baseline Privacybetween the two CMTSs.

In order to quickly switch the CM to from the Working CMTS to theProtect CMTS, the CM needs to pre-initialization and perform occasionalranging with the Protect CMTS. The operation of the CM with the ProtectCMTS can be classified as four states:

-   -   Pre-Initialization: The CM will partially initialize with the        Protect CMTS.    -   Occasional Ranging: The CM will perform occasional ranging with        the Protect CMTS    -   Failure Switch: The CM will perform final initialization with        the Protect CMTS during failure    -   Normal Operation: Standard operation    -   For Pre-Initialization, the challenges are:        -   CM may not get a primary SID assigned;        -   CM's current IP address may not be supported by the Protect            CMTS;        -   CM must not initialize Baseline Privacy operations if the CM            is provisioned to run Baseline Privacy.

The solution is for the CM to perform final initialization with ProtectCMTS during failure.

For Occasional Ranging, the challenges are:

-   -   CM may not get a primary SID assigned;    -   each ranging opportunity must be quick enough;    -   each region must be large enough to account for variation in        delays between any two CMs.

The solution is to allocate a quick contention ranging opportunity toModem Occasional Ranging SID for Protect CMTS. This is similar inconcept to the Initial Maintenance IE.

For Failure Switch, the challenges are:

-   -   CM must first does Failure Ranging which has the same problems        as Occasional Ranging, than final initial with Protect CMTS.

The solution is to allocate a much more quicker contention rangingopportunity to Modem Failure Ranging SID for Protect CMTS. This issimiliar in concept to the Initial Maintenance IE. The CM gets a PrimarySID assigned in RNG-RSP with Ranging Status=success message. The CM mustinvoke DHCP mechanisms to obtain an IP address and must initializeBaseline Privacy operations if the CM is provisioned to run BaselinePrivacy.

For Normal Operation, the challenge is:

-   -   Protect CMTS may not support that many CMs at normal operation.

The solution is for the Protect CMTS to send the CM back to its WorkingCMTS or some other Working CMTS.

D. Other Embodiments

Setting working and protection paths, as described above, has anotherapplication beyond merely providing redundancy. Typically installing newsoftware on a cable network is very problematic, mainly because thetypes of bugs and how to remedy them are unknown ahead of time. Thus,there must a period of service time in which the network may experiencesignificant problems associated with the new software's bugs. In fact,the network performance can be so poor, that the old software isreinstalled. By providing a protection path, the new software can betested by some of the cable modems without disrupting service throughthe working path for most cable modems. Thus, the new software and itsaffects on the cable network can be characterized before it is used foractual service.

While the discussion to this point has focused on a redundancytechnology for cable networks, the technology of the present inventionmay be applied to any shared-access network having a plurality of hostsor nodes which share at least one channel for communicating with atleast one “head-end” in the network. Examples of shared-access networksinclude, in addition to cable networks, wireless networks, Ethernet,etc. In the cable network, the plurality of nodes represents a pluralityof cable modems that communicate with at least one CMTS at thecentralized termination system using at least one shared-access upstreamand downstream channel.

In general, the methods and apparatus described above may be implementedon a protection device (e.g., a router) for providing redundancy in anetwork having (1) a working device (e.g., another router) that providesnormal service to a host and (2) the protection device which takes overservice to the host should service from the working device fail. Suchgeneral methods may include the following sequence: (a) pre-registeringthe host with the protection device before or after it registers withthe working device; and (b) assuming a protection state in which theprotection device can take over service of the host should its servicewith the working device fail. Generally, such methods (and associatedapparatus) will be particularly valuable in the context of telephonyservice.

In the wireless system (e.g., represented by FIG. 9) the plurality ofnodes or hosts corresponds to the plurality of wireless nodes 950 whichuse at least one shared access channel to communicate with at least oneaccess control system 922 located at the head end of the wirelesssystem.

As shown in FIG. 9, the wireless system includes a central terminationsystem (or head end) 920. The head end includes a working accesscontroller or access control system (ACS) 922 which communicates with aplurality of wireless nodes 950, and coordinates access between each ofthe wireless nodes and the head end 920. The access controller 922 mayinclude memory and at least one processor. In a specific embodiment, thefunction of the access controller 922 is analogous to that of the CMTSdescribed above with respect to cable modem networks. It may serve as arouter as well.

The head end 920 communicates with a plurality of wireless nodes 950 viaany one of a plurality of wireless transmitting and receiving devices910. As shown in FIG. 9, for example, the plurality of wirelesstransmitting and receiving devices 910 may include satellite basestations 902, orbital satellites 906, radio towers 904, etc.

In a specific embodiment which is analogous to that of cable modemnetworks, the head end 920 of the wireless computer system communicateswith the plurality of nodes 950 via one or more downlink channels 907and one or more uplink channels 909. Each downlink channel 907 is abroadcast-type channel utilized by the head end to communicate with anassociated group of wireless nodes within the wireless network. Theuplink channel 909 is a shared-access channel, which is utilized by agroup of wireless nodes (analogous to cable modems) to communicate withthe head end 920.

The working access controller 922 stores registration parameters for thevarious nodes that it services. The access controller 922 may also storethe IP addresses for nodes that it services while being backed up by aprotection access controller 923. These IP addresses are also stored byprotection access controller 922 to allow a smooth transition in serviceshould working access controller 922 fail.

In a specific embodiment of the present invention, the registrationprocess and information is similar to that of the cable network CMTSsdescribed above. Moreover, the technique of the present invention forcutover using a single IP address for both the working and protectionaccess controllers may be implemented in wireless system 900.

The wireless devices or nodes 950 may include any one of a number ofwireless transmitting/receiving devices. For example, a satellite dish952 may be used to communicate with the head end 920 via the uplink anddownlink channels. The satellite dish may, in turn, be connected to alocal area network (LAN) 930 which, may be further connected to one ormore computer systems 932. Another wireless device may be aportable/wireless computer system 954, which is able to transmit andreceive information to the head end via uplink and downlink channels 907and 909. Other wireless devices 956 may include, for example, wirelesstelephones, handheld computing devices, etc.

In specific embodiments where the uplink and downlink channels withinthe wireless system 900 are utilized in a manner similar to that of theupstream and downstream channels of a cable modem network, theabove-described redundancy methods may easily be implemented in wirelesssystem 900 using the detailed description of the present inventionprovided herein. Moreover, the technique of the present invention may beeasily implemented in any computer network which uses shared accesschannels for communicating between a centralized computing system andone or more remote nodes.

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. For example, while ranging was described above, othertechniques for causing modems to transmit signals at predefinedfrequencies and amplitudes may be employed.

1. A method implemented on a working CMTS for providing redundancy to a cable network in which the working CMTS has a subnet of IP addresses for assigning to its cable modems and the working CMTS provides normal service to a cable modem and a protection CMTS, which has its own subnet of IP addresses, takes over service to the cable modem should service from the working CMTS become unavailable, the method comprising: (a) participating in registration of the cable modem to allow servicing of the cable modem by the working CMTS; and (b) obtaining an IP address for the cable modem, which IP address is obtained from an address space outside the subnet of the working CMTS, wherein the IP address is used in communications with both the working CMTS and the protection CMTS, whereby taking over service to the cable modem is performed without re-registering the cable modem or obtaining a new IP address for the cable modem and comprises requesting that the cable modem adjust at least one of the following parameters: transmission power, transmission frequency, and transmission time slots.
 2. The method of claim 1, wherein the working CMTS is a routing CMTS.
 3. The method of claim 1, wherein the address space from which the IP address for the cable modem is obtained is outside of the subnet for the protection CMTS.
 4. The method of claim 1, further comprising communicating the IP address for the cable modem to the protection CMTS.
 5. The method of claim 1, further comprising injecting a host route to the cable modem into a routing protocol.
 6. The method of claim 5, wherein injecting the host route comprises providing the host route to one or more aggregation routers servicing the cable network.
 7. The method of claim 1, wherein participating in registration of the cable modem comprises specifying at least one of a transmission power, a transmission frequency, and transmission time slots at which the cable modem is to communicate with the working CMTS.
 8. The method of claim 1, wherein participating in registration of the cable modem comprises participating in a process to assign the IP address for the cable modem.
 9. The method of claim 8, wherein the process to assign the IP address for the cable modem is DHCP, and wherein the working CMTS obtains the IP address while performing a DHCP relay function.
 10. The method claim 1, further comprising: (c) determining that the working CMTS's service to the cable modem has or will become unavailable; and (d) notifying at least one of the cable modem and the protection CMTS that the cable modem should obtain service from the protection CMTS.
 11. The method of claim 1, wherein the service provided to the cable modem includes telephony service.
 12. A computer program product comprising a machine readable medium on which is provided program instructions for performing a method implemented on a working CMTS, which method provides redundancy for a cable network in which the working CMTS has a subnet of IP addresses for assigning to its cable modems and the working CMTS provides normal service to a cable modem and a protection CMTS, which has its own subnet of IP addresses, takes over service to the cable modem should service from the working CMTS become unavailable, the program instructions comprising instructions for: (a) participating in registration of the cable modem to allow servicing of the cable modem by the working CMTS; and (b) obtaining an IP address for the cable modem, which IP address is obtained from an address space outside of the subnet of the working CMTS, wherein the IP address is used for communications with both the working CMTS and the protection CMTS, whereby taking over service to the cable modem is performed without re-registering the cable modem or obtaining a new IP address for the cable modem, and comprises requesting that the cable modem adjust at least one of the following parameters: transmission power, transmission frequency, and transmission time slots.
 13. The computer program product of claim 12, further comprising program instructions for communicating the IP address for the cable modem to the protection CMTS.
 14. The computer program product of claim 12, further comprising instructions for injecting a host route to the cable modem into a routing protocol.
 15. The computer program product of claim 14, wherein the instructions for injecting a host route comprise instructions for providing the host route to one or more aggregation routers servicing the cable network.
 16. The computer program product of claim 12, wherein the instructions for participating in registration of the cable modem comprise instructions for participating in a process to assign the IP address for the cable modem.
 17. The computer program product of claim 16, wherein the instructions for obtaining the IP address for the cable modem comprise instructions for performing a DHCP relay function.
 18. The computer program product of claim 12, further comprising instructions for: (c) determining that the working CMTS's service to the cable modem has or will become unavailable; and (d) notifying at least one of the cable modem and the protection CMTS that the cable modem should obtain service from the protection CMTS.
 19. A CMTS designed or configured to act as a working CMTS for a cable network including the working CMTS which has a subnet of IP addresses for assigning to its cable modems and provides normal service to a cable modem and a protection CMTS which has its own subnet of IP addresses and takes over service to the cable modem should service from the working CMTS become unavailable, the CMTS comprising: (a) one or more processors; (b) memory in communication with at least one of the one or more processors; and (c) wherein at least one of the one or more processors is configured to store registration data for the cable modem in the memory, and wherein the registration data specifies an IP address for the cable modem, which IP address resides in an address space outside the subnet of the working CMTS, and wherein the IP address is used for communication with both the working CMTS and the protection CMTS, whereby at least one of the one or more processors is configured to allow taking over service from another CMTS without re-registering the cable modem or obtaining a new IP address for the cable modem, and in doing so to request that the cable modem adjust at least one of the following parameters: transmission power, transmission frequency, and transmission time slots.
 20. The CMTS of claim 19, wherein the CMTS is a routing CMTS.
 21. The CMTS of claim 19, wherein at least one of the one or more processors is designed or configured to inject a host route to the cable modem into a routing protocol.
 22. The CMTS of claim 19, wherein at least one of the one or more processors is configured to notify at least one of the cable modem and the protection CMTS that the cable modem should obtain service from the protection CMTS when the working CMTS's service to the cable modem has become unavailable.
 23. A method implemented on a protection CMTS for providing redundancy for a cable network having a working CMTS that has a subnet of IP addresses for assigning to its cable modems and provides normal service to a cable modem and the protection CMTS which has its own subnet of IP addresses and takes over service to the cable modem should service from the working CMTS become unavailable, the method comprising: (a) determining that the working CMTS's service to the cable modem has or will become unavailable; and (b) taking over service to the cable modem while using an IP address for the cable modem that was also used while the working CMTS was servicing the cable modem, whereby taking over service to the cable modem is performed without re-registering the cable modem or obtaining a new IP address for the cable modem and comprises requesting that the cable modem adjust at least one of the following parameters: transmission power, transmission frequency, and transmission time slots, wherein the IP address for the cable modem resides in an address space located outside the subnet for the working CMTS and also outside the subnet for the protection CMTS.
 24. The method of claim 23, wherein determining that the working CMTS's service to the cable modem has or will become unavailable comprises receiving a notification to that effect from at least one of the cable modem and the working CMTS.
 25. The method of claim 23, fully comprising injecting a host route to the cable modem, through the protection CMTS, into a routing protocol.
 26. The method of claim 25, wherein injecting a host route comprises providing the host route to one or more aggregation routers servicing the cable network.
 27. A method comprising: using a first address to enable a first device to communicate with a first CMTS which has a subnet of IP addresses for assigning to its cable modems wherein the first address is assigned by the first CMTS and the first address is outside the first CMTS's subnet; attaching to a second CMTS which has its own subnet of IP addresses; and using said first address to enable said first device to communicate with said second CMTS and thereby allowing the second device to take over service from said first device, whereby taking over service to the cable modem is performed without re-registering the cable modem or obtaining a new IP address for the cable modem and comprises requesting that the cable modem adjust at least one of the following parameters: transmission power, transmission frequency, and transmission time slots.
 28. The method of claim 27 further including detaching from said first CMTS.
 29. The method of claim 27 wherein said first address is an IP address.
 30. The method of claim 29 wherein said first device is a cable modem, and wherein said assigning includes assigning said first IP address to said cable modem.
 31. The method of claim 27 wherein said first address is associated with said first device, and wherein said method includes injecting a host route for accessing said first device associated with said first address into a router protocol, said host route including routing information which specifies that said first device may be accessed via said first CMTS using said first address.
 32. The method of claim 31 further including injecting a second host route for accessing said first device into said router protocol upon determining that said first device has attached to said second CMTS, said second host route including routing information which specifies that said first device may be accessed via said second CMTS using said first address.
 33. The method of claim 32 wherein said first device is a cable modem.
 34. The method of claim 27 wherein said address assigning includes receiving said first address from DHCP server.
 35. The method of claim 34 further including: submitting an address request for communicating with said second CMTS; and receiving said first address from said DHCP device for communicating with said second CMTS.
 36. The method of claim 27 further including: detecting a problem in the communication path between said first device and said first CMTS; and in response to detection of said problem, detaching from said first CMTS and attaching to a second CMTS using said first address.
 37. The method of claim 1, wherein notifying at least one of the cable modem and the protection CMTS that the cable modem should obtain service from the protection CMTS comprises sending a downstream channel change request to the cable modem. 